Disconnectable submerged buoy mooring device comprising clamping dogs

ABSTRACT

A mooring assembly includes: a mooring buoy having a central axis, an upper ring portion providing an upper abutment surface; a mooring structure including a cavity with a wall receiving the mooring buoy; and a buoy locking system near the cavity engaging with the upper ring portion for locking the mooring buoy to the turret mooring structure and including at least two locking devices attached to the mooring structure, each locking device having a locking dog connected to a force member for axial displacement and exerting an upward force on the upper abutment surface. The mooring buoy includes a lower abutment surface. The locking system includes an engagement member below the locking dog, engaging with the lower abutment surface and exerting a downward force. The buoy at positions axially above the upper abutment surface situated at an axial clearance from the cavity wall.

FIELD OF THE INVENTION

This invention relates to a mooring assembly comprising:

-   -   a mooring buoy having a central axis, an upper ring portion        providing an upper abutment surface,    -   a mooring structure comprising a cavity with a cavity wall        arranged for receiving the mooring buoy, and

a buoy locking system arranged near the cavity for engaging with theupper ring portion for locking the mooring buoy to the turret mooringstructure and comprising at least two locking devices attached to themooring structure, each locking device having a locking dog connected toa force member for axial displacement and exertion of an upward force onthe upper abutment surface.

The invention also relates to a locking device, to a structure, such asa pre-assembled turret, and to a vessel comprising a mooring structureand a buoy locking system according to the invention.

BACKGROUND OF THE INVENTION

Turret mooring assemblies are known on hydrocarbon production and/orprocessing vessels and comprise a mooring buoy and a turret mooringstructure. The mooring buoy is anchored to the seabed with anchoringlegs. The turret mooring structure, provided on a vessel, has a cavityfor receiving the mooring buoy and one or more buoy locking devices forlocking the mooring buoy in the cavity.

The turret mooring structure may be an internal turret mooring structureor an external turret mooring structure. An internal turret mooringstructure is provided within the hull of the vessel, in a so-called moonpool. The cavity is formed as an opening at or near the bottom of thevessel, facing downwards. An external turret mooring structure isprovided outside the hull of the vessel. The external turret is fixedwith suitable connection members at an outboard position at the bow orstern of the vessel.

The mooring buoy may be moved up and down, i.e. from a storage positionat a safe distance below the water surface (e.g. 30-200 meters) to amooring position close to or at the surface of the water where it can bereceived by the cavity.

The turret mooring structure itself is connected to the vessel, but isrotatable with respect to the vessel, allowing the vessel to weathervaneunder influence of wind, waves, currents and drifting ice. The turretmooring system may be disconnected and reconnected when needed, therebyproviding a disconnectable turret mooring system.

The turret mooring system comprises a fluid transfer system to allowtransportation of hydrocarbons, such as oil or gas, for instance byestablishing a flow path between the vessel and a subsea well via theturret mooring system and the mooring buoy.

The turret mooring structure may comprise a first part of the fluidtransfer system and the mooring buoy may comprise a second part of thefluid transfer system. The turret mooring structure may comprise aturret manifold and the mooring buoy may comprise a buoy manifold, bothmanifolds each comprising at least one conduit. The turret and buoymanifold are matching such that conduits of the turret manifold can beconnected to corresponding conduits of the buoy manifold of the fluidtransfer system to establish a flow path.

During the connection of the mooring buoy to the turret mooringstructure, the mooring buoy is locked in a fixed position with respectto the turret mooring structure inside the cavity.

According to state of the art in turret mooring systems, centering ofthe mooring boy in relation to the turret is carried out while lockingthe mooring buoy into the cavity of the turret mooring structure.

International patent applications WO1993011030-WO1993011035 discloselocking mechanisms of a turret mooring structure comprising a pluralityof locking fingers distributed around an annular locking shoulder of thebuoy, for releasable locking of the outer member of the mooring buoy inthe receiving space. The mechanism comprises a pair of locking dogswhich are actuated by a hydraulic system and are rotatable abouthorizontal axes at diametrically opposite sides of the receiving space.If desired, more than two locking dogs may be provided. The hydraulicactuators for operation of the locking dogs may for example be hydrauliccylinders. When activating the locking dogs these will pivot in avertical plane to engagement with the downwards facing abutment edge ofthe upper cone member.

International patent application WO2001089919 discloses a mechanism forreleasably locking of an element in relation to a base, especially forthe locking of a buoy in a downwardly open receiving space in a floatingvessel. The mechanism comprises a rotatably mounted locking arm which ispivotable between a release position and a locking position in which anabutment edge on the locking arm is in engagement with an abutment edgeon the element to be locked, a linkage which is connected between thelocking arm and the base and which, in the locking position, is in aself-locking over-centre position, and a driving means for actuation ofthe linkage. The linkage comprises a length-adjustable first link which,from an initial position with the locking arm close to its lockingposition, is arranged to be extended to thereby pivot the locking armadditionally to a final locking position, for achieving a desiredpreloading force in the engagement between the abutment edges of thelocking arm and the element. The first link preferably is a hydrauliccylinder having a piston rod of which one end is connected to thelocking arm.

From WO 2010/081826 a turret mooring assembly according to the preambleof claim 1 is known. The upper rim portion of the known riser supportingbuoy is engaged by a number of hydraulically actuated locking members,that exert an upward force on the buoy. The upper rim portion of theknown buoy is clamped in an upward direction against an abutment ring ofthe receiving cavity. The upper rim portion of the buoy is very stiffand loads on the buoy are al transferred into the turret via the lockingmembers. Cyclic loading of the locking members can lead to reducedfatigue life and malfunctioning of the locking members. Furthermore, inthe known locking structure the deflections between the upper part ofthe buoy and the receiving cavity may be relatively large, resulting indifficulties in applying a water-tight seal between the top of the buoyand the cavity wall.

It is therefore an object of the present invention to provide a mooringsystem that overcomes one or more of the disadvantages from the priorart. It is in particular an object of the invention to provide a mooringsystem in which the loads on the locking device are reduced.

It is again an object of the invention to provide a mooring system thatallows effectively applying a water-tight seal between the buoy and thecavity wall.

SUMMARY OF THE INVENTION

Hereto, the mooring system according to the invention is characterizedin that:

-   -   the mooring buoy comprises a lower abutment surface placed at an        axial distance below the upper abutment surface,    -   the locking system comprises an engagement member situated        axially below the locking dog, for engaging with the lower        abutment surface and exerting a downward force thereon,    -   the buoy being at positions axially above the upper abutment        surface situated at an axial clearance (D) from the cavity wall        such that axial forces on the buoy are transferred to the turret        mooring structure substantially only at the positions of the        locking dog and the engagement member.

Because the buoy—in particular the upper ring portion of thebuoy—according to the invention is pulled against the engagement member,an elastic preloading of the buoy is achieved and a relatively long loadpath is established. When downward forces on the buoy increase, theseincreased forces have the effect of first reducing the preload contactbetween the buoy and the engagement member, only a smaller portion ofthe added load being taken up by the locking dog. This means thatinternal portions of the locking system are subject to reduced fatigueload amplitudes. The varying preload force between the locking systemand the buoy does not significantly contribute to fatigue effects on thebuoy or on the locking system.

In an embodiment of a mooring assembly according to the invention, theengagement member comprises a horizontal ring-shaped surface having aring-shaped sealing member thereon for preventing ingress of water alongthe space between the lower abutment surface and the engagement member.

The decreasing preloading on the engagement member results in theengagement member remaining in contact with the lower abutment surfaceof the buoy so that the seal effectively operates under static sealconditions which can be easily maintained. Also under very heavyvertical loads, the separation of the lower abutment surface of the buoyand the engagement member on the turret will not be more than a few mm,which gap can be bridged by elastic expansion of the seal.

The ring-shaped surface can be situated at a larger radial distance fromthe central axis than the locking dog, such that a stepped constructionis formed on which the annular seal can be accommodated.

The locking dog may be rotatable around a horizontal shaft, the forcemember comprising an axially movable hydraulic jack. Each hydraulic jackmay be connected to a high pressure unit, such as an accumulator orpressure pump that is of compact size to power a single locking devicein a standalone manner, independently from the other locking devices.The high pressure unit may be used as a backup for providing rapiddisconnection of the locking devices in case of an emergency.

In a preferred embodiment, the high pressure unit of each locking deviceis mounted on the hydraulic jack of each respective hydraulic power unitfor following pivoting movements of the hydraulic jack. In this mannerno flexible hoses need be applied for supplying high pressure fluidduring expansion and contraction of the piston rod of the hydraulicjack.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying schematic drawings in which correspondingreference symbols indicate corresponding parts, and in which:

FIG. 1a shows a schematic drawing of a turret mooring system provided ona vessel;

FIG. 1b shows an enlargement of the selected box in FIG. 1A;

FIG. 2 shows a perspective view of an embodiment of a locking deviceaccording to the present invention;

FIG. 3 shows a cross-section of the locking device of FIG. 2;

FIG. 4a schematically shows the locking device during a first stage ofpositioning of the buoy into the turret;

FIG. 4b schematically shows the locking device during a last stage ofpositioning of the buoy into the turret;

FIGS. 5a and 5b schematically show a locking device in accordance withan embodiment of the invention;

FIG. 6 schematically shows a locking device according to an embodimentof the invention;

DETAILED DESCRIPTION

In the following figures, the same reference numerals refer to similaror identical components in each of the figures.

FIGS. 1a-1b show schematic drawings of a turret mooring system 1provided on a vessel 2, which for example could be a floating productionunit (FPU) or floating production storage and offloading (FPSO) unit orfloating storage and offloading (FSO) unit. The vessel 2 comprises ahull 16 having near a bottom 17 a moon pool 18. A lifting device 26 isplaced on the turret mooring structure 3 comprising a cable 19, shown inFIGS. 1a-b and 2, that extends through a central shaft 24 provided inthe mooring buoy 6. In addition, the vessel 2 comprises a turret mooringsystem 1, wherein the turret mooring system 1 is rotatably suspendedfrom the hull 16 of the vessel 2. The turret mooring system 1 comprisesa turret mooring structure 3 within the moon pool 18.

A turret bearing system 21 connects and aligns the turret mooringstructure 3 with respect the vessel 2. The turret mooring system 1 is asa whole rotationally suspended from the vessel 2. The turret mooringsystem 1 can rotate with respect to the vessel 2 to allow the vessel 2to weathervane after connection to the mooring buoy 6 or to orientatethe turret mooring system 1 with respect to the mooring buoy 6, withoutthe need to reposition the vessel 2.

In addition, the turret mooring system 1 may comprise an intermediateconnection member 4. Such an intermediate connection member 4 isarranged to be rotated together with the mooring buoy 6 with respect tothe turret mooring structure 3, i.e. after locking the mooring buoy 6inside the cavity 5, so that the fluid piping of the turret manifoldpiping and the buoy manifold piping can be aligned. The intermediateconnection member 4 is positioned in between the turret mooringstructure 3 and the mooring buoy 6, if present. After disconnecting themooring buoy 6, the intermediate connection member 4 remains attached tothe turret mooring structure 3.

The turret mooring structure 3 comprises a cavity 5 for receiving themooring buoy 6. In an alternative embodiment (not shown), the cavity 5could be attached directly to the intermediate connection member 4, ifpresent. The mooring buoy 6 carries an anchoring system 27 which maycomprise at least 1 anchoring leg 22 that is connected to a seabed 23.The mooring buoy 6 is receivable in the cavity 5 for coupling with theturret mooring structure 3.

The turret mooring structure 3 comprises a buoy locking system 7,comprising a number of buoy locking devices 10, that may becircumferentially distributed around the cavity, for locking the mooringbuoy 6 inside the cavity 5. An embodiment of the buoy locking device 10is described with reference to FIGS. 2-7. The buoy locking system 7 isonly shown schematically in FIGS. 1a -b.

The turret mooring structure 3 comprises a turret manifold 8 that can beconnected, after alignment, to a corresponding buoy manifold 9 toestablish a fluid flow path between the turret mooring structure 3 andthe mooring buoy 6.

When the mooring buoy 6 enters the cavity 5, the mooring buoy 6 ispre-centered into the cavity 5, due to its conical shape and fenders 11on the inside of the cavity 5, and due to the pulling tension in thereconnection winch cable of the lifting device 26. The fenders 11 onlyserve to maintain a predetermined radial position of the buoy from thecavity wall and in no way impair up and down movement of the buoy 6inside the cavity 5. At the top side of the buoy 6 a gap with a width Dof is maintained between the buoy and the cavity wall, which may amountto one or a few mm. Substantially no vertically upward forces aretransferred from the buoy to the cavity for parts of the buoy that aresituated above the buoy locking devices 10.

When approaching the locking devices 10, the upper ring portion 200 ofthe buoy, lifted-up by a winch, comes into contact with the lockingdevices 10. When centering is completed, the locking devices are closedand clamp the upper ring portion 200 of the buoy in the turret mooringstructure 3.

After alignment of the fluid piping manifolds and the locking of theturret with regard to the connected buoy 6, a fluid transfer path can beestablished between the turret and buoy manifold.

-   -   FIG. 2 shows a perspective view of an embodiment of a locking        device according to the present invention.

The locking device 10 comprises a first support frame 100 and a secondsupport frame 102. Further the locking device 10 comprises a locking dog106 that is actuated by a main hydraulic jack 110.

In this embodiment, the first support frame 100 is substantiallyT-shaped and connected to the turret mooring structure by fixed bottomand side supports 120 and 122 which are located at two substantiallyperpendicular ends of the T-shaped first support frame. The connectionbetween the first support frame 100 and the fixed supports may be byshafts 121 and 123 respectively.

The second support frame 102 is a substantially oblong frame which has afirst end that is rotatably connected to the first support frame by acommon shaft 104, which is located in the first support frame at somevertical distance above the fixed bottom support 120.

The locking dog 106 is rotatably connected to the first support frame100 on the common shaft 104. The centering dog 108 is connected to thefirst support frame 100 on a dedicated shaft 125 which is located at avertical distance above the common shaft.

The locking dog 106 is arranged on a free end 124 of the first supportframe 100, i.e., the end of the first support frame that is notconnected to the fixed bottom or side supports 120, 122.

A second end of the second support frame 102 is rotatably connected toone end of the main hydraulic jack 110 by means of a shaft 128.

The opposite end of the hydraulic jack 110 is rotatably connected to thelocking dog 106 through a hinge 132. The arrangement of the hydraulicjack and the locking dog is described in more detail with reference toFIG. 3. A hydraulic power unit (HPU) 90 is mounted on the hydraulic jack110, so as to be pivotable together with the jack, as can be clearlyseen in FIG. 3.

Additionally, the locking device comprises on the second support frame102 a backup release (hydraulic) jack 140, which provides a releasableconnection between the second support frame and the first support frameat a location adjacent to the fixed side support 122.

One or more of shafts 104, 121, 122, 125, 128 of the locking device 10are preferably provided with low friction bushes.

The hydraulic jack on each individual locking device can be operated bya dedicated HPU, with the HPU being integrated within the locking devicestructure (i.e. mounted onto the jack and fluid connected to thecylinder via a rigid duct instead of fluid connection over a distancevia a flexible duct). This has the advantage of having a standalonelocking device forming a one piece assembly which is complete and onlyneeds to be installed on site.

-   -   FIG. 3 shows a cross-section of the locking device of FIG. 2.

The hydraulic jack 110 is rotatably connected to an extension arm 109 ofthe locking dog 106 through the hinge 132, such that a change of thelength of the hydraulic jack causes a rotation of the locking dog 106around common shaft 104. In FIG. 3, the hydraulic jack is shown in itsextended position, with the locking dog 106 in a clamping position ofthe mooring buoy (not shown). In this clamping position, the buoy isclamped in a vertical direction by the locking dog 106 against the lowerengagement member 107 (see FIG. 4c ; FIG. 6) or 108 (see FIG. 5b ) or onthe turret without the top surface 201 of the buoy abutting against thesupport frame 100 or any other parts of the cavity wall such that apreloading of the buoy against the relatively elastic lower engagementmember is effected and peak loads are prevented from acting on thelocking dog 6.

The backup release jack 140 is shown in FIG. 3 in the connected positionhaving a release pin 141 attached to the second support frame andpositioned in a locking hole 142 attached to the first support frame toform a releasable connection 141, 142. The backup release jack isarranged as a releasable lock that can break the releasable connectionin case of a malfunction of the locking device (or an emergency) duringthe clamping position. By releasing the releasable connection the secondsupport frame 102 can rotate with respect to the first support framearound the common shaft. Since the rotation axis (common shaft) of thesecond support frame coincides with the rotation axis 104 of the lockingdog 106, the locking dog will rotate accordingly to an open position soas to release the buoy.

In FIG. 4a schematically the locking device 10 is shown during a firststage of positioning of the buoy 6 into the turret mooring structure 3.

The mooring buoy 6 comprises a top ring portion 200 which has an upperedge or surface 201 and a lower edge 202.

FIG. 4b schematically shows the locking device during connection of thebuoy 6 into the cavity 5 of the turret in accordance with an embodimentof the invention.

In this embodiment, the mooring buoy 6 comprises a ring portion 200 at atop part of the buoy that is received in the turret. The ring portion isprovided with a first abutment surface 202 and a second abutment surface203 placed at a distance from and in parallel with the first abutmentsurface.

The buoy locking system 7 comprises at least one locking device 10 thatin a first support frame 100 comprises a rotatable locking dog 106 and alower engagement element 107. Within the locking device 10 the lockingdog 106 and the lower engagement element 107 are positioned at avertical distance from each other.

The rotatable locking dog 106 is arranged for engagement with the upperabutment surface 202 provided on the ring portion 200 of the mooringbuoy 6. The upper abutment surface is collar shaped and positioned in anupper region of the ring portion. Thus a contacting surface of thelocking dog can engage the first abutment surface by moving upwards.

The lower abutment surface 203 is arranged below the upper abutmentsurface 202. The engagement element 107 of the locking device can bepassive and can engage with the lower abutment surface 203 of themooring buoy 6, the upward movement of the locking dog effectivelypulling the lower abutment surface 203 up to a contacting surface of theengagement element 107. An annular sealing element 109 may be attachedto the engagement element 107 for preventing of water ingress along theengagement element. The engagement element 107 may be embodied as afixed bumper.

In the embodiment shown in FIG. 5a , the mooring buoy 6 comprises a ringportion 200 at a top part of the buoy that is received in the turret.The ring portion is provided with a first abutment surface 203 and asecond abutment surface 203 placed at a distance from and in parallelwith the first abutment surface.

The buoy locking system 7 comprises at least one locking device 10 thatin a first support frame 100 comprises a rotatable upper locking dog 106and a lower centering dog element 208. Within the locking device 10 thelocking dog 106 and a lower centering dog 208 are positioned at avertical distance from each other. The lower centering dog 208 isarranged for rotation around a horizontal axis 209 between a lowerposition and an upper position. In the upper position the lowercentering dog 208 is configured to be blocked from further upwardrotation around the axis 209.

The rotatable locking dog 106 is arranged for engagement with the firstabutment surface 202 provided on the ring portion 200 of the mooringbuoy 6. The first abutment surface is collar shaped and positioned in anupper region of the ring portion. Thus a contacting surface of thelocking dog 106 can engage the first abutment surface by moving upwards.

Before contacting the second abutment surface 203 of the buoy 6, thelower centering dog 208 is in a freely pivoting open position,configured to contact and engage with the second abutment surface 203and after contact to be rotated upward with the upward moving buoy 6.

During the upward movement of the second abutment surface 203, the lowercentering dog 208 rotates upwards, until the upper position is reachedand further rotation is blocked. In that upper position the upwardmovement of the buoy surface is stopped.

The upward movement of the locking dog 106 may enlarge the distancebetween the locking dog and the lower centering dog 208, thuseffectively pulling the second abutment surface up to a contactingsurface of the lower centering dog 208. In this manner the buoy isclamped to the locking device 10.

In the clamped position a distance between the contacting surface of therotatable locking dog 106 and the contacting surface of the lowercentering dog 208 corresponds substantially with the distance betweenthe second abutment surface 203 and the first abutment surface 202 onthe mooring buoy.

The contacting surfaces of the locking device may exert adequate forceson the first and second abutment surfaces 202, 203 to generate aclamping force for holding the mooring buoy 6 in position.

FIG. 6 schematically shows a locking device in accordance with anembodiment of the invention.

In this embodiment, the buoy locking system 7 comprises at least onelocking device 10, with first support frame 100 that is connected to theturret mooring structure 205 via a support member 206 and whichcomprises the rotatable locking dog 106. The buoy 6 has an annularabutment surface 203 that is, via a flexible annular seal 204, pulledagainst the turret structure 205 by upward movement of the locking dog106.

1. Mooring assembly comprising: a mooring buoy (6) having a centralaxis, an upper ring portion (200) providing an upper abutment surface(202), a mooring structure (3) comprising a cavity (5) with a cavitywall arranged for receiving the mooring buoy (6), and a buoy lockingsystem (7) arranged near the cavity (5) for engaging with the upper ringportion (200) for locking the mooring buoy (6) to the turret mooringstructure (6) and comprising at least two locking devices (10) attachedto the mooring structure (3), each locking device (10) having a lockingdog (106) connected to a force member (110) for axial displacement andexertion of an upward force on the upper abutment surface (202) wherein,the mooring buoy (6) comprises a lower abutment surface (203) placed atan axial distance below the upper abutment surface (202), the lockingsystem (7) comprises an engagement member (107,208) situated axiallybelow the locking dog (106), for engaging with the lower abutmentsurface (203) and exerting a downward force thereon, the buoy being atpositions (201) axially above the upper abutment surface (202) situatedat an axial clearance (D) from the cavity wall such that axial forces onthe buoy are transferred to the turret mooring structure (6)substantially only at the positions of the locking dog (106) and theengagement member (107,208).
 2. Mooring assembly according to claim 1,wherein the engagement member (107) comprises a horizontal ring-shapedsurface having a ring-shaped sealing member thereon for preventingingress of water along the space between the lower abutment surface(203) and the engagement member (107).
 3. Mooring assembly according toclaim 2, wherein the ring-shaped surface is situated at a larger radialdistance from the central axis than the locking dog (106).
 4. Mooringassembly according to claim 1, wherein the locking dog (106) isrotatable around a horizontal shaft (104), the force member comprisingan axially movable hydraulic jack (110).
 5. Mooring assembly accordingto claim 4, a hydraulic power unit being mounted on the hydraulic jack(110) for supplying high pressure fluid to the hydraulic jack and forfollowing pivoting movements of the hydraulic jack.
 6. A locking device(10) for use in a buoy locking system, having frame with a pivotablelocking dog (106) and with a hydraulic jack (110) attached to the frameand to the locking dog (110) for pivoting the locking dog, a hydraulicpower unit being mounted on the hydraulic jack (110) for supplying highpressure fluid to the hydraulic jack and for following pivotingmovements of the hydraulic jack.
 7. Structure having a mooring systemand a buoy locking system according to claim
 1. 8. Vessel having amooring system and a buoy locking system according to claim
 1. 9.Hydrocarbon production system comprising a vessel according to claim 8.10. Mooring assembly according to claim 2, wherein the locking dog (106)is rotatable around a horizontal shaft (104), the force membercomprising an axially movable hydraulic jack (110).
 11. Mooring assemblyaccording to claim 3, wherein the locking dog (106) is rotatable arounda horizontal shaft (104), the force member comprising an axially movablehydraulic jack (110).
 12. Structure having a mooring system and a buoylocking system according to claim
 2. 13. Structure having a mooringsystem and a buoy locking system according to claim
 3. 14. Structurehaving a mooring system and a buoy locking system according to claim 4.15. Structure having a mooring system and a buoy locking systemaccording to claim
 5. 16. Structure having a mooring system and a buoylocking device according to claim
 6. 17. Vessel having a mooring systemand a buoy locking system according to claim
 2. 18. Vessel having amooring system and a buoy locking system according to claim
 3. 19.Vessel having a mooring system and a buoy locking system according toclaim
 4. 20. Vessel having a mooring system and a buoy locking systemaccording to claim 5.